KR102657497B1 - Electrolyte for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to an electrolyte solution for a lithium secondary battery containing a lithium salt, a nitrogen compound, and an organic solvent. It contains lithium bis(trifluoromethanesulfonyl)imide as the lithium salt and an ether-based solvent as the organic solvent. , it has excellent oxidation stability and can exhibit excellent storage stability at high temperatures.

Description

Electrolyte for lithium secondary batteries and lithium secondary batteries containing the same {ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to an electrolyte for lithium secondary batteries and a lithium secondary battery containing the same.

As the scope of use of lithium secondary batteries expands not only to portable electronic devices but also to electric vehicles (EV) and electric storage systems (ESS), the demand for lithium secondary batteries with high capacity, high energy density, and long life is increasing. there is.

Among various lithium secondary batteries, lithium-sulfur batteries use sulfur-based materials containing sulfur-sulfur bonds as the positive electrode active material, and lithium metal, carbon-based materials in which insertion/deinsertion of lithium ions occurs, or lithium It is a battery system that uses silicon or tin, which forms an alloy, as a negative electrode active material.

Sulfur, the main material of the positive electrode active material in lithium-sulfur batteries, has the advantage of having a low weight per atom, being easy to supply due to abundant resources, being inexpensive, non-toxic, and environmentally friendly.

In addition, lithium-sulfur batteries use a conversion reaction of lithium ions and sulfur at the anode (S ₈ +16Li ⁺ +16e ^- → The theoretical specific capacity from 8Li ₂ S) reaches 1,675 mAh/g, and when lithium metal is used as the cathode, the theoretical energy density is 2,600 Wh/kg. This is similar to other battery systems currently being studied (Ni-MH battery: 450 Wh/kg, Li-FeS battery: 480 Wh/kg, Li-MnO2 battery: 1,000 Wh/kg, Na-S battery: 800 Wh/kg) and Because it has a very high value compared to the theoretical energy density of a lithium-ion battery (250 Wh/kg), it is attracting attention as a high-capacity, eco-friendly, and low-cost lithium secondary battery among secondary batteries being developed to date.

When a lithium-sulfur battery is discharged, a reduction reaction occurs where sulfur accepts electrons at the positive electrode, and an oxidation reaction occurs where lithium is ionized at the negative electrode.

When these _lithium -sulfur batteries are discharged, lithium polysulfide (Li ₂ S It accelerates deterioration and causes a shuttle reaction during the charging process, greatly reducing charge and discharge efficiency. In addition, lithium metal used as a negative electrode continuously reacts with the electrolyte, promoting decomposition of the lithium salt and additives in the electrolyte.

To solve this problem, Republic of Korea Patent Publication No. 10-2016-0037084 blocks lithium polysulfide from melting by coating graphene on carbon nanotube aggregates containing sulfur, and improves the conductivity and sulfur content of the sulfur-carbon nanotube composite. It is disclosed that the loading amount can be increased.

However, the problem of the lithium-sulfur battery becomes more severe at high temperatures and electrolyte decomposition is accelerated, while the prior art does not disclose improvement of the problems at high temperatures.

Therefore, there is a need to develop an electrolyte with excellent stability in order to operate a lithium-sulfur battery in a high temperature environment.

Accordingly, the present inventors conducted various studies to solve the above problems, and as a result, lithium bis(trifluoromethanesulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide was found as a lithium salt of electrolyte for lithium secondary batteries. The present invention was completed by confirming that when lithium bis(fluorosulfonyl)imide is included, oxidation stability and storage ability at high temperatures are improved, and that lithium secondary batteries containing it have improved lifespan characteristics at high temperatures. .

Therefore, the purpose of the present invention is to provide an electrolyte solution for lithium secondary batteries that has excellent oxidation stability and improved storage ability at high temperatures.

In addition, the present invention aims to provide a lithium secondary battery that can achieve improved lifespan characteristics at high temperatures by including the electrolyte for lithium secondary batteries.

In order to achieve the above purpose,

According to one aspect of the present invention, electrolyte solutions for lithium secondary batteries of the following embodiments are provided.

The electrolyte for a lithium secondary battery according to the first embodiment,

It includes a lithium salt, a nitrogen compound, and an organic solvent. The lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, and the organic solvent includes an ether-based solvent.

According to the second embodiment, in the first embodiment,

The lithium bis(trifluoromethanesulfonyl)imide may be included in an amount of 20 mol% or more based on the total number of moles of the lithium salt.

According to a third embodiment, in the first or second embodiment,

The molar concentration of the lithium salt may be 0.1 to 4M.

According to a fourth embodiment, in any one of the first to third embodiments,

The lithium salt further includes lithium bis(fluorosulfonyl)imide, and the molar concentration of the lithium bis(trifluoromethanesulfonyl)imide is the molar concentration of lithium bis(fluorosulfonyl)imide. It can be the same or higher.

According to a fifth embodiment, in any one of the first to fourth embodiments,

The ether-based solvent may be included in an amount of 80% by volume or more based on the total volume of the organic solvent.

According to a sixth embodiment, in any one of the first to fifth embodiments,

The ether-based solvent may include linear ether, cyclic ether, or a mixture thereof.

According to a seventh embodiment, in any one of the first to sixth embodiments,

The linear ether includes dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethylmethyl ether, ethylpropyl ether, ethyltertbutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, Diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol di. Vinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether and ethylene glycol ethyl methyl ether. It may include one or more types selected from the group consisting of.

According to the eighth embodiment, in any one of the first to seventh embodiments,

The cyclic ethers include 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl- 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1 ,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl- The group consisting of 1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,4-dimethoxy benzene and isosorbide dimethyl ether. It may include one or more types selected from.

According to the ninth embodiment, in any one of the first to eighth embodiments,

The organic solvent may have a solubility of 2 g/100 g or more for the nitrogen compound at room temperature based on 100 g of the organic solvent.

According to a tenth embodiment, in any one of the first to ninth embodiments,

The room temperature may be in the temperature range of 20°C to 35°C.

According to an eleventh embodiment, in any one of the first to tenth embodiments,

The organic solvent may not contain a carbonate-based solvent.

According to a twelfth embodiment, in any one of the first to eleventh embodiments,

The carbonate-based solvent includes dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, It may be 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, halides thereof, or a mixture of two or more thereof.

According to a thirteenth embodiment, in any one of the first to twelfth embodiments,

The nitrogen compound may include a nitric acid compound or a nitrite-based compound.

According to a fourteenth embodiment, in any one of the first to thirteenth embodiments,

The nitrogen compound may be included in an amount of 2% to 10% by weight based on the total weight of the electrolyte for the lithium secondary battery.

According to a fifteenth embodiment, in any one of the first to fourteenth embodiments,

When the electrolyte solution for a lithium secondary battery is stored at a temperature of 45°C or higher, more than 90% by weight of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide may remain.

According to the sixteenth embodiment, in any one of the first to eleventh embodiments,

When the electrolyte for a lithium secondary battery is stored for 4 weeks at a temperature of 45°C or higher, 90% to 98% by weight of the lithium bis(trifluoromethanesulfonyl)imide may remain compared to the initial weight of the lithium bis(trifluoromethanesulfonyl)imide.

According to the seventeenth embodiment, in any one of the first to sixteenth embodiments,

The storage temperature may be 45°C to 65°C.

According to another aspect of the present invention, lithium secondary batteries of the following embodiments are provided.

The lithium secondary battery according to the 18th embodiment,

anode; cathode; A separator interposed between the anode and the cathode; and a lithium secondary battery comprising an electrolyte solution, wherein the electrolyte solution is an electrolyte solution according to any one of the first to seventeenth embodiments.

According to the 19th embodiment, in the 18th embodiment,

The positive electrode may include a sulfur-containing compound as a positive electrode active material.

According to the 20th embodiment, in the 18th or 19th embodiment,

The sulfur-containing compound is inorganic sulfur (S ₈ ), lithium polysulfide (Li ₂ Sn, 1≤n≤8), carbon sulfur polymer (C ₂ Sx)m, 2.5≤x≤50, 2≤m) or any of these. It may contain a mixture of two or more.

According to the twenty-first embodiment, in any one of the eighteenth to twentieth embodiments,

The negative electrode may include lithium metal, lithium alloy, or a mixture thereof as a negative electrode active material.

According to the 22nd embodiment, in any one of the 18th to 21st embodiments,

The lithium secondary battery may be a coin-type battery or a pouch-type battery.

The electrolyte solution for lithium secondary batteries of the present invention has high oxidation stability and has excellent storage stability at high temperatures.

In addition, a lithium secondary battery containing the electrolyte for a lithium secondary battery of the present invention has excellent lifespan characteristics at high temperatures.

Figure 1 is a photograph observing the oxidation stability of the electrolyte solutions for lithium-sulfur batteries of Examples 1 to 4 and Comparative Example 1.
Figure 2 is a graph measuring the capacity retention rate of a lithium-sulfur battery after high temperature storage.
Figure 3 is a graph measuring the lifespan characteristics of a coin cell type lithium-sulfur battery at high temperature (45°C).
Figure 4 is a graph measuring the lifespan characteristics of a pouch cell type lithium-sulfur battery at high temperature (45°C).
Figure 5 is a photograph evaluating the solubility of nitrogen compounds depending on the type of organic solvent according to Experimental Example 6 in the present specification (left: Example 4, right: Comparative Example 2).

Hereinafter, the present invention will be described in more detail.

Terms or words used in this specification and claims should not be construed as limited to their common or dictionary meanings, and the inventor may appropriately define the concept of terms in order to explain his or her invention in the best way. It must be interpreted with meaning and concept consistent with the technical idea of the present invention based on the principle that it is.

The terms used in the present invention are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In the present invention, terms such as 'include' or 'have' are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but are not intended to indicate the presence of one or more other features or It should be understood that this does not exclude in advance the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

The term “composite” as used herein refers to a material that combines two or more materials to form physically and chemically different phases while exhibiting more effective functions.

The term "polysulfide" used in this specification means "polysulfide _ion (S _x ^2- , ^{x -} = 8, 6, 4 _, 2)" and "lithium polysulfide (Li _{2 S} It is a concept that includes “8, 6, 4, 2)”.

Lithium secondary batteries, especially lithium-sulfur batteries, have a problem of poor stability because the lithium negative electrode and the electrolyte continuously react during charging and discharging, thereby promoting decomposition of the lithium salt. As the temperature increases, the decomposition of the lithium salt accelerates.

Lithium bis(fluorosulfonyl)imide, which was conventionally used as a lithium salt, did not effectively suppress the decomposition of lithium salt in the electrolyte solution, and in particular, it failed to suppress the decomposition of lithium salt, which accelerates in a high temperature environment of 45℃ or higher, leading to poor stability. there was.

One aspect of the present invention seeks to solve the above problems.

Electrolyte for lithium secondary batteries

Therefore, the present invention relates to an electrolyte solution for lithium secondary batteries that can solve the above problems,

The electrolyte for lithium secondary batteries of the present invention is

Contains lithium salts, nitrogen compounds and organic solvents,

The lithium salt includes lithium bis(trifluoromethanesulfonyl)imide (LiTFSI),

The organic solvent includes an ether-based solvent.

The lithium bis(trifluoromethanesulfonyl)imide has excellent oxidation stability and high temperature stability, and decomposition can be suppressed even if the electrolyte solution continuously reacts with lithium metal, which is the negative electrode of a lithium secondary battery. Therefore, in the present invention, when the lithium bis(trifluoromethanesulfonyl)imide is included as the lithium salt, an electrolyte for a lithium secondary battery is provided that not only has excellent oxidation stability but also excellent storage stability, especially at high temperatures. However, the mechanism of the present invention is not limited thereto.

In the present invention, storage stability at high temperatures means that when the electrolyte solution is applied to a lithium secondary battery, the lithium salt is not decomposed and at least some content remains even at a temperature of 45°C or higher. For example, when the electrolyte solution is applied to a lithium secondary battery, the lithium salt applied to the electrolyte solution may maintain a content of 90% by weight or more relative to the initial weight at a temperature of 45°C or higher.

In one embodiment of the present invention, the lithium bis(trifluoromethanesulfonyl)imide may be included in an amount of, for example, 20 mol% or more based on the total number of moles of the lithium salt. For example, based on the total number of moles of the lithium salt, 20 mol% to 100 mol%, 25 mol% to 95 mol%, 30 mol% to 90 mol%, 40 mol% to 80 mol%, 45 mol% to 75 mol%. , may be included in an amount of 50 mol% to 75 mol%, or 60 mol% to 75 mol%. When the content of the lithium bis(trifluoromethanesulfonyl)imide is within the above-mentioned range, it can exhibit an advantageous effect in terms of exerting the above-described effect by the lithium bis(trifluoromethanesulfonyl)imide. , the present invention is not limited thereto.

In another embodiment of the present invention, the lithium salt may further include other types of lithium salts in addition to the lithium bis(trifluoromethanesulfonyl)imide.

In one embodiment of the present invention, other types of lithium salts other than lithium bis(trifluoromethanesulfonyl)imide may include, for example, lithium bis(fluorosulfonyl)imide.

In one embodiment of the present invention, the electrolyte for a lithium secondary battery further includes lithium bis(trifluoromethanesulfonyl)imide and lithium bis(trifluorosulfonyl)imide as the lithium salt. effects can be obtained. When the lithium bis(trifluoromethanesulfonyl)imide is further included, in order to obtain the above effect, the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is lithium bis(fluorosulfonyl)imide. It may be advantageous to have a molar concentration equal to or higher than the molar concentration of lithium bis(trifluoromethanesulfonyl)imide. However, the present invention is not limited thereto.

In one embodiment of the present invention, the lithium bis(trifluoromethanesulfonyl)imide and the lithium bis(fluorosulfonyl)imide are, for example, 1:5 to 5:1, 1:3 to 3. :1, 1:2 to 2:1, 1:1.5 to 1.5:1, 1:1 to 1.5:1, 1:1 to 2:1, 1:1 to 3:1, 1.5:1 to 5:1 Alternatively, it may be included in a molar ratio of 2:1 to 3:1. When the lithium bis(trifluoromethanesulfonyl)imide and the lithium bis(fluorosulfonyl)imide are included in the above-described molar ratio, a beneficial effect may be shown in terms of stability of the electrolyte solution, but the present invention is not limited thereto. That is not the case.

In one embodiment of the present invention, the concentration of the lithium salt may be appropriately determined considering ionic conductivity, solubility, etc., and may be, for example, 0.1 to 4M, preferably 0.5 to 2M. When the concentration of the lithium salt is in the above-mentioned range, it is advantageous to secure ionic conductivity suitable for battery operation, or it can exhibit an appropriate viscosity of the electrolyte solution, which can have an advantageous effect in terms of improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself. However, the present invention is not limited to this.

In addition to the lithium salt, the nitrogen compound not only improves the electrical conductivity of the lithium secondary battery by dissolving in the electrolyte of the lithium secondary battery and providing ions, but also improves the electrical conductivity of the lithium secondary battery when the electrolyte for the lithium secondary battery is used in a lithium-sulfur battery. This is to improve the lifespan characteristics of. Specifically, the nitrogen compound prevents irreversible consumption of polysulfide by suppressing the reduction reaction of polysulfide that occurs during the charging and discharging process of a lithium-sulfur battery, for example, although its effectiveness is not limited thereto. It can act to improve the performance of lithium-sulfur batteries.

In one embodiment of the present invention, the nitrogen compound is not particularly limited as long as it forms a stable film on the lithium metal electrode, which is the negative electrode of a lithium secondary battery, specifically a lithium-sulfur battery, and has the effect of improving charge and discharge efficiency. For example, it may be a nitric acid compound, a nitrite-based compound, or a mixture thereof.

In one embodiment of the present invention, the nitrogen compound is, for example, lithium nitrate (LiNO ₃ ), potassium nitrate (KNO ₃ ), cesium nitrate (CsNO ₃ ), barium nitrate (Ba(NO ₃ ) ₂ ), and ammonium nitrate. Inorganic nitric acid or nitrite compounds such as (NH ₄ NO ₃ ), lithium nitrite (LiNO ₂ ), potassium nitrite (KNO ₂ ), cesium nitrite (CsNO ₂ ), and ammonium nitrite (NH ₄ NO ₂ ); Organic nitric acids such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite. or nitrous acid compounds; It may be selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof, preferably May contain lithium nitrate.

In one embodiment of the present invention, the nitrogen compound is, for example, 1% by weight to 10% by weight, 2% by weight to 10% by weight, or 3% by weight to 10% by weight, based on the total weight of the electrolyte solution for the lithium secondary battery. Specifically, it may be included in an amount of 3% to 8% by weight, 3% to 6% by weight, or 3% to 5% by weight, but is not limited thereto. When the nitrogen compound is included in the above-described content, it may exhibit a more advantageous effect in terms of improving the electrical conductivity of the electrolyte by the nitrogen compound and suppressing the reduction of polysulfide when used in a lithium-sulfur battery, but the present invention is limited thereto. That is not the case.

The organic solvent is a medium through which ions involved in the electrochemical reaction of a lithium secondary battery can move, and is used to dissolve the lithium salt and/or the nitrogen compound.

In the present invention, the organic solvent includes an ether-based solvent.

In one embodiment of the present invention, the organic solvent is 80 vol% or more of the ether-based solvent, for example, 85 vol% to 100 vol%, 90 vol% to 100 vol%, 95 vol% based on the total volume of the organic solvent. It may be included in an amount of from 100% by volume, 98% by volume to 100% by volume, 90% by volume to 98% by volume, or 90% by volume to 95% by volume. When the content of the ether-based solvent is within the above-described range based on the total volume of the organic solvent, a beneficial effect may be exhibited in terms of solubility of the lithium salt and nitrogen compound, but the present invention is not limited thereto.

In one embodiment of the present invention, the ether-based solvent may include linear ether, cyclic ether, or a mixture thereof.

In one embodiment of the present invention, the linear ether is, for example, dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethylmethyl ether, ethylpropyl ether, ethyltertbutyl ether, dimethyl Toxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether , Diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, Diethylene glycol ethylmethyl ether, Diethylene glycol isopropylmethyl ether, Diethylene glycol butylmethyl ether, di It may include at least one member selected from the group consisting of ethylene glycol tertbutyl ethyl ether and ethylene glycol ethyl methyl ether. Preferably, it may include one or more selected from the group consisting of dimethyl ether, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and more preferably. May contain dimethoxyethane.

In one embodiment of the present invention, the cyclic ether is, for example, 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl -1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2 -Ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3 -dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,4- It may include at least one member selected from the group consisting of dimethoxy benzene and isosorbide dimethyl ether. Preferably, it may include at least one selected from the group consisting of 2-methylfuran, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran and 2,5-dimethyltetrahydrofuran, and more Preferably, it may contain 2-methylfuran.

In one embodiment of the present invention, the organic solvent may include dimethoxyethane and 2-methylfuran.

Additionally, the organic solvent may contain linear ether and cyclic ether in a volume ratio of 95:5 to 5:95, preferably 95:5 to 50:50, and most preferably 90:10 to 70:30. In the present invention, the volume ratio corresponds to the ratio of "volume % of linear ether":"volume % of cyclic ether" in the ether-based solvent.

In one embodiment of the present invention, the organic solvent may exhibit excellent solubility for the nitrogen compound at room temperature. As described above, in order to effectively exhibit the above-described effect caused by the nitrogen compound, the organic solvent must be able to sufficiently dissolve the nitrogen compound.

In one embodiment of the present invention, the organic solvent is 2 g/100 g or more, for example, 2 g/100 g to 20 g/, based on 100 g of the organic solvent, for example, at room temperature. 100 g, 3 g/100 g to 20 g/100g, 3 g/100 g to 15 g/100 g, 5 g/100 g to 15 g/100 g, 5 g/100 g to 10 g/100 g, or It may exhibit a solubility of 7 g/100 g to 10 g/100 g. If the organic solvent can exhibit the above-described solubility for the nitrogen compound, it can have a beneficial effect in terms of reducing the waste of the nitrogen compound and improving the performance of the lithium secondary battery by the nitrogen compound, but the present invention is limited to this. It doesn't work.

In one embodiment of the present invention, the 'room temperature' may refer to a temperature range of, for example, 20°C to 35°C, specifically, a temperature range of 25°C to 30°C.

In one embodiment of the present invention, other organic solvents other than ether-based solvents may be used as long as the organic solvent can dissolve the nitrogen compound. For example, organic solvents used in the electrolyte solution of conventional lithium secondary batteries include esters, amides, linear carbonates, and cyclic carbonates in addition to the ether-based solvents. In one embodiment of the present invention, the organic solvent is the ether. In addition to the system solvent, it may further include an organic solvent used in the electrolyte solution of the conventional lithium secondary battery described above. However, preferably, in terms of solubility of the nitrogen compound, the electrolyte solution for a lithium secondary battery includes the organic solvent and the carbonate-based solvent. It may be desirable not to include .

In one embodiment of the present invention, the ester is, for example, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valero. Any one selected from the group consisting of lactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more of these may be used, but is not limited thereto.

In one embodiment of the present invention, the linear carbonate is, for example, any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate and ethylpropyl carbonate or any of these. A mixture of two or more types may be typically used, but is not limited thereto.

In one embodiment of the present invention, the cyclic carbonate is, for example, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3 -There is any one selected from the group consisting of pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halogenates thereof, or a mixture of two or more thereof. These halides include, for example, fluoroethylene carbonate, but are not limited thereto.

In another embodiment of the present invention, because the carbonate-based solvent does not dissolve the nitrogen compound or exhibits low solubility, the organic solvent may not include the carbonate-based solvent.

In one embodiment of the present invention, the organic solvent may contain a very small amount of the carbonate-based solvent such that the carbonate-based solvent does not affect the solubility of the nitrogen compound. For example, the organic solvent may contain a very small amount of the carbonate-based solvent. When included, the content of the carbonate-based solvent is 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.5% by weight or less, or 0% by weight (i.e., not included at all) based on the total weight of the electrolyte for the lithium secondary battery. may not).

The electrolyte solution for lithium secondary batteries of the present invention may specifically be an electrolyte solution for lithium-sulfur batteries.

The electrolyte solution for a lithium secondary battery of the present invention has excellent oxidation stability as it contains the lithium salt, and can suppress decomposition of the lithium salt, showing excellent stability, especially at high temperatures.

More specifically, even when stored at a high temperature of 60°C or higher, decomposition of the lithium salt rarely occurs, and a lithium-sulfur battery containing it can show improved lifespan characteristics at a temperature of 45°C or higher.

In one embodiment of the present invention, the electrolyte for a lithium secondary battery, for example, when stored at high temperature, the lithium bis(trifluoromethanesulfonyl)imide is not decomposed at all, or only a small amount is decomposed, and the lithium bis(trifluoromethanesulfonyl)imide is decomposed at 90% by weight compared to the initial weight. It may be to maintain the weight above.

Specifically, the electrolyte solution for a lithium secondary battery is stored at a high temperature, for example, 45°C or higher, specifically 45°C to 65°C, 50°C to 60°C, or 55°C to 60°C for 2 weeks or more, for example, 2 weeks to 2 weeks. When stored for 12 weeks, 2 weeks to 10 weeks, 3 weeks to 8 weeks, 3 weeks to 6 weeks, for example, 4 weeks to 5 weeks, the lithium bis(trifluoromethanesulfonyl)imide is 90% by weight compared to the initial weight. The weight may be maintained at or above, for example, 90 to 100% by weight, 90 to 98% by weight, or 93 to 96% by weight.

In one embodiment of the present invention, the content of lithium bis(trifluoromethanesulfonyl)imide before and after storage of the electrolyte for a lithium secondary battery is determined by nuclear magnetic resonance analysis (NMR) of the electrolyte, It can be measured by known content analysis methods such as ion chromatography and immediate constituent analysis.

Lithium secondary battery

In addition, the present invention is an anode; cathode; A separator interposed between the anode and the cathode; and a lithium secondary battery comprising an electrolyte, wherein the electrolyte is the same as the electrolyte of the present invention described above.

anode

The positive electrode may include a positive electrode current collector and a positive electrode active material layer applied to one or both sides of the positive electrode current collector.

The positive electrode current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, surface treatment of copper or stainless steel with carbon, nickel, silver, etc., aluminum-cadmium alloy, etc. can be used.

The positive electrode current collector can strengthen the bonding force with the positive electrode active material by forming fine irregularities on its surface, and can be used in various forms such as film, sheet, foil, mesh, net, porous material, foam, and non-woven fabric.

The positive electrode active material layer includes a positive electrode active material and may further include a conductive material, binder, and additives.

The positive electrode active material includes a porous carbon material and a sulfur-carbon composite containing sulfur on at least some of the inner and outer surfaces of the porous carbon material. Since sulfur contained in the positive electrode active material does not conduct electricity alone, it is used in combination with a conductive material such as carbon material. Accordingly, the sulfur is contained in the form of a sulfur-carbon complex.

Therefore, the lithium secondary battery of the present invention may be a lithium-sulfur battery.

The sulfur may include one or more types selected from the group consisting of elemental sulfur (S ₈ ) and sulfur compounds. The positive electrode active material is selected from the group consisting of inorganic sulfur, Li ₂ Sn (n≥1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers ((C ₂ S _x ) _n , x=2.5 to 50, n≥2). It may include one or more selected types. Preferably, the sulfur may be inorganic sulfur.

The sulfur-carbon composite not only provides a framework in which the above-described sulfur can be fixed uniformly and stably, but also includes a porous carbon material to compensate for the low electrical conductivity of sulfur so that the electrochemical reaction can proceed smoothly.

The porous carbon material can generally be manufactured by carbonizing precursors of various carbon materials. The porous carbon material includes irregular pores inside, the average diameter of the pores is in the range of 1 to 200 nm, and the porosity may be in the range of 10 to 90 vol% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if it exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it difficult to apply in the electrode manufacturing process. Not desirable.

In one embodiment of the present invention, the 'average pore diameter' can be measured according to a method known in the art for measuring the pore diameter of a porous material, and the measurement method is not particularly limited. For example, the diameter of the pore may be measured according to scanning electron microscopy (SEM), field emission electron microscopy (laser diffraction method), or laser diffraction method. Measurement using the laser diffraction method may, for example, use a commercially available ray diffraction particle size measurement device (for example, Microtrac MT 3000).

In one embodiment of the present invention, the 'porosity' refers to the ratio of the volume occupied by pores to the total volume in a structure, and vol% is used as its unit, and is interchanged with terms such as porosity and porosity. You can use it. In the present invention, the measurement of the porosity is not particularly limited, and according to one embodiment of the present invention, for example, BET (Brunauer-Emmett-Teller) measurement using nitrogen gas or mercury permeation (Hg porosimeter) and Can be measured according to ASTM D2873.

The porous carbon material can be used without limitation as long as it is spherical, rod-shaped, needle-shaped, plate-shaped, tube-shaped or bulk-shaped and is commonly used in lithium-sulfur batteries.

The porous carbon material may be any one that has a porous structure or has a high specific surface area and is commonly used in the art. For example, the porous carbon material includes graphite; graphene; Carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Carbon nanotubes (CNTs), such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); Carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); It may be one or more types selected from the group consisting of graphite such as natural graphite, artificial graphite, expanded graphite, and activated carbon, but is not limited thereto. Preferably, the porous carbon material may be a carbon nanotube.

In the sulfur-carbon composite according to the present invention, the sulfur is located on at least one of the inner and outer surfaces of the porous carbon material, for example, less than 100% of the entire inner and outer surfaces of the porous carbon material, preferably 1 to 100%. It may be present in the range of 95%, more preferably 40 to 96%. When the sulfur is present within the above range on the inner and outer surfaces of the porous carbon material, the maximum effect can be achieved in terms of electron transfer area and wettability with electrolyte. Specifically, since the sulfur is thinly and evenly impregnated on the inner and outer surfaces of the porous carbon material in the above-mentioned range, the electron transfer contact area can be increased during the charging and discharging process. If the sulfur is located in 100% of the entire inner and outer surface of the porous carbon material, the porous carbon material is completely covered with sulfur, and the wettability to the electrolyte is reduced and the contact property is lowered, so that electrons are not transferred, resulting in an electrochemical reaction. You will not be able to participate.

The sulfur-carbon composite may include, for example, 65% by weight or more of sulfur, specifically 65 to 90% by weight, 70 to 85% by weight, or 72 to 80% by weight, based on 100% by weight of the sulfur-carbon composite. You can. When the sulfur content is within the above-mentioned range, advantageous effects can be shown in terms of improving battery performance and securing battery capacity, but the present invention is not limited thereto.

The method for producing the sulfur-carbon composite of the present invention is not particularly limited in the present invention, and methods commonly used in the art may be used. For example, a method of simply mixing the sulfur and the porous carbon material and then heat treating them to form a composite may be used.

In addition to the composition described above, the positive electrode active material may further include one or more selected from transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur.

The transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, etc. are included, the group 2 elements include Al, Ga, In, Ti, etc., and the group ₃ elements may include Ge, Sn, Pb, etc.

In the positive electrode for a lithium secondary battery of the present invention, the positive electrode active material is, for example, 80% by weight or more, specifically 80% by weight to 100% by weight, more specifically 85% by weight to 98% by weight, or 80% by weight, based on the total weight of the positive electrode active material layer. It may be included in weight% to 95% by weight. The content of the positive electrode active material may have a lower limit of 70% by weight or more or 85% by weight or more, and an upper limit of the content of the positive electrode active material may be 99% by weight or less or 90% by weight or less, based on 100% by weight of the total positive electrode active material layer. The content of the positive electrode active material can be set as a combination of the lower limit and the upper limit. If the content of the cathode active material is less than the above range, the relative content of auxiliary materials such as conductive material and binder increases, and the content of the cathode active material decreases, making it difficult to implement a battery with high capacity and high energy density. Conversely, if it exceeds the above range, it will be described later. There is a problem that the physical properties of the electrode are deteriorated due to a relatively insufficient content of the conductive material or binder.

The conductive material is a material that electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material. It is physically distinct from the carbon contained in the sulfur-carbon complex. Any electrode component that has conductivity can be used without limitation.

For example, the conductive material includes carbon black such as Super-P, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon black; Carbon derivatives such as carbon nanotubes and fullerenes; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Alternatively, conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole can be used alone or in combination.

The content of the conductive material may be 1 to 10% by weight based on the total weight of the positive electrode active material. If the content of the conductive material is less than the above range, electron transfer between the positive electrode active material and the current collector is not easy, and thus voltage and capacity decrease. On the contrary, if it exceeds the above range, the ratio of the positive electrode active material may be relatively reduced and the total energy (charge amount) of the battery may decrease, so it is desirable to determine the appropriate content within the above range.

The binder maintains the positive electrode active material in the positive electrode current collector and organically connects the positive active materials to increase the binding force between them. All binders known in the art can be used.

For example, the binder may be polyvinylidene fluoride (PVdF), a polyvinylidene fluoride-based polymer containing at least one vinylidene fluoride as a repeating unit, polytetrafluoroethylene (PTFE), or these. A fluororesin binder containing a mixture of two or more of the following; Rubber-based binders including styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; Acrylic binder; Cellulose-based binders including carboxymethyl cellulose (CMC), starch, hydroxypropylcellulose, and regenerated cellulose; polyalcohol-based binder; Polyolefin-based binders including polyethylene and polypropylene; polyimide-based binder; polyester-based binder; and a silane-based binder; one, two or more types of mixtures or copolymers selected from the group consisting of may be used.

The content of the binder may be 1 to 10% by weight based on the total weight of the positive electrode active material layer. If the content of the binder is less than the above range, the physical properties of the positive electrode may deteriorate and the positive electrode active material and the conductive material may fall off. If the content exceeds the above range, the ratio of the positive active material and the conductive material in the positive electrode may be relatively reduced, thereby reducing battery capacity. It is desirable to determine the appropriate content within the above-mentioned range.

In the present invention, the method of manufacturing the positive electrode for a lithium secondary battery is not particularly limited, and known methods or various methods modifying the same can be used by those skilled in the art.

For example, the positive electrode for a lithium secondary battery may be manufactured by preparing a positive electrode slurry composition containing the composition described above and then applying it to at least one surface of the positive electrode current collector to form the positive electrode active material layer.

The positive electrode slurry composition includes the positive electrode active material as described above, and may further include a binder, a conductive material, and a solvent.

The solvent that can uniformly disperse the positive electrode active material is used. As such a solvent, water is most preferable as an aqueous solvent, and in this case, the water may be distilled water or deionized water. However, it is not necessarily limited to this, and if necessary, lower alcohol that can be easily mixed with water can be used. The lower alcohols include methanol, ethanol, propanol, isopropanol, and butanol, and preferably, they can be mixed with water.

The content of the solvent may be contained at a level that can facilitate coating, and the specific content varies depending on the application method and device.

The positive electrode slurry composition may, if necessary, additionally contain materials commonly used in the relevant technical field for the purpose of improving its function. Examples include viscosity modifiers, fluidizers, fillers, etc.

The method of applying the positive electrode slurry composition is not particularly limited in the present invention, and includes methods such as doctor blade, die casting, comma coating, and screen printing. You can. Additionally, the positive electrode slurry may be applied on the positive electrode current collector by pressing or lamination after molding on a separate substrate.

After the application, a drying process may be performed to remove the solvent. The drying process is performed at a temperature and time sufficient to remove the solvent, and the conditions may vary depending on the type of solvent, so the present invention is not particularly limited. Examples include drying using warm air, hot air, or low humidity air, vacuum drying, and drying using irradiation with (far) infrared rays and electron beams. The drying speed is usually adjusted to remove the solvent as quickly as possible within a speed range that does not cause cracks in the positive electrode active material layer or peeling of the positive electrode active material layer from the positive electrode current collector due to stress concentration.

Additionally, the density of the positive electrode active material in the positive electrode can be increased by pressing the current collector after drying. Press methods include methods such as mold press and roll press.

The porosity of the positive electrode, specifically the positive electrode active material layer, manufactured using the composition and manufacturing method described above may be 50 to 80 vol%, specifically 60 to 75 vol%. If the porosity of the positive electrode is less than 50 vol%, the filling degree of the positive electrode slurry composition including the positive electrode active material, conductive material, and binder becomes too high, so that sufficient electrolyte to exhibit ionic conduction and/or electrical conduction between the positive electrode active materials is required. As this cannot be maintained, the output characteristics or cycle characteristics of the battery may deteriorate, and there is a problem of severe overvoltage and reduction in discharge capacity of the battery. On the other hand, if the porosity of the positive electrode exceeds 80 vol% and has an excessively high porosity, the physical and electrical connection with the current collector is lowered, which causes problems of lowered adhesion and difficult reactions, and the increased porosity is filled with electrolyte, causing the battery to There is a problem that the energy density may be lowered, so adjust appropriately within the above range.

cathode

The negative electrode may include a negative electrode current collector and a negative electrode active material layer applied to one or both sides of the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

The negative electrode current collector is for supporting the negative electrode active material layer, and is the same as described for the positive electrode current collector.

The negative electrode active material layer may include a conductive material, a binder, etc. in addition to the negative electrode active material. At this time, the conductive material and binder are as described above.

The negative electrode active material is a material capable of reversibly intercalating or deintercalating lithium (Li ⁺ ), a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, lithium metal, or a lithium alloy. It can be included.

The material capable of reversibly inserting or de-inserting lithium ions (Li ⁺ ) may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The material that can react with the lithium ion (Li ⁺ ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy includes, for example, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium ( It may be an alloy of a metal selected from the group consisting of Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

Preferably, the negative electrode active material may be lithium metal, and specifically, may be in the form of a lithium metal thin film or lithium metal powder.

separator

The separator separates or insulates the positive and negative electrodes from each other and enables the transport of lithium ions between the positive and negative electrodes. The separator may be made of a porous non-conductive or insulating material. If it is usually used as a separator in a lithium secondary battery, it may be a special membrane. It can be used without restrictions. This separator may be an independent member such as a film, or may be a coating layer added to the anode and/or cathode.

The separator preferably has low resistance to ion movement in the electrolyte and has excellent moisture retention capacity for the electrolyte.

The separator may be made of a porous substrate. Any porous substrate commonly used in secondary batteries can be used, and porous polymer films can be used alone or by stacking them, for example, a high melting point film. A non-woven fabric made of glass fiber, polyethylene terephthalate fiber, etc., or a polyolefin-based porous membrane can be used, but is not limited thereto.

The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate may be made of polyolefin such as polyethylene or polypropylene, polyester such as polyethyleneterephthalate or polybutyleneterephthalate, or polyamide. (polyamide), polyacetal, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylene sulfide ( polyphenylenesulfide, polyethylenenaphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon It may include one or more materials selected from the group consisting of (nylon), poly(p-phenylene benzobisoxazole), and polyarylate.

The thickness of the porous substrate is not particularly limited, but may be 1 to 100 ㎛, preferably 5 to 50 ㎛. The thickness range of the porous substrate is not limited to the above-mentioned range, but if the thickness is too thin than the above-mentioned lower limit, the mechanical properties may decrease and the separator may be easily damaged during battery use.

The average diameter and pore size of the pores present in the porous substrate are also not particularly limited, but may be 0.001 to 50 ㎛ and 10 to 95 vol%, respectively.

The lithium secondary battery according to the present invention is capable of lamination (stack) and folding processes of separators and electrodes in addition to the general winding process.

The shape of the lithium secondary battery is not particularly limited, and may have various shapes such as cylindrical, stacked, coin, or pouch.

Hereinafter, preferred embodiments are presented to aid understanding of the present invention. However, the following examples are merely illustrative of the present invention, and it is clear to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention. It is natural that such variations and modifications fall within the scope of the attached patent claims.

<Manufacture of electrolyte for lithium-sulfur batteries>

Examples 1 to 4 and Comparative Example 1.

An electrolyte solution for a lithium-sulfur battery was prepared with the composition shown in Table 1 below.

lithium salt organic solvent nitrogen compounds type density Example 1 LiTFSI 0.1875M 2-MeF:DME
(2:8 (v/v)) LINO ₃ LiFSI 0.5625M Example 2 LiTFSI 0.375M LiFSI 0.375M Example 3 LiTFSI 0.5625M LiFSI 0.1875M Example 4 LiTFSI 0.75M LiFSI - Comparative Example 1 LiTFSI - LiFSI 0.75M

The nitrogen compound was contained at 5% by weight based on the total weight of the electrolyte for lithium-sulfur batteries, where 2-MeF is 2-methylfuran and DME is dimethoxyethane.

Experimental Example 1. Evaluation of oxidation stability of electrolyte for lithium-sulfur batteries

The oxidation stability of the electrolyte solutions for lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 was evaluated.

The oxidation stability evaluation was performed by exposing the electrolyte solutions for lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 to air (20% oxygen) at 25°C and 1 atm, storing the vial caps closed, and measuring the electrolyte solution 24 hours later. The degree of browning was evaluated by visual observation, and the results are shown in Figure 1.

Referring to Figure 1, the electrolyte solutions for lithium-sulfur batteries prepared in Examples 1 to 4 and Comparative Example 1 were all observed to brown due to oxidation by oxygen in the air present inside the vial, but lithium bis (trifluorolyte) It was confirmed that the higher the concentration of lomethanesulfonyl)imide (LiTFSI), the weaker the degree of browning.

Therefore, it can be seen that the higher the concentration of lithium bis(trifluoromethanesulfonyl)imide as the lithium salt of the electrolyte solution, the better the oxidation stability.

Experimental Example 2. Evaluation of high temperature (60°C) stability of electrolyte solution for lithium-sulfur batteries

The electrolyte solutions for lithium-sulfur batteries prepared in Examples 2 and 3 and Comparative Example 1 were stored in a sealed state in a high temperature chamber at 60°C for 4 weeks, and then the lithium salt remaining in the electrolyte solution was quantitatively analyzed to evaluate high temperature stability. .

Quantitative analysis of lithium salt was performed by extracting the organic solvent using THF, followed by NMR analysis using isotopes and ion chromatography (IC) analysis, and the results are shown in Table 2 below. Table 2 below shows the weight of the remaining lithium salt calculated as a percentage compared to the initial weight of the lithium salt.

LiTFSI LiFSI Example 2 98% 0% Example 3 96% 0% Comparative Example 1 - 0%

The results in Table 2 show that lithium bis(fluorosulfonyl)imide (LiFSI) did not remain and was all decomposed. However, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) showed a result of remaining up to 98%. Therefore, lithium bis(trifluoromethanesulfonyl)imide does not decompose at high temperature (60℃) and does not decompose at high temperature (60℃). , it was confirmed that it was very stable.

Experimental Example 3. Evaluation of capacity retention rate of lithium-sulfur battery after storage at high temperature (60°C)

A positive electrode slurry composition was prepared by mixing 95% by weight of sulfur-carbon composite (S:C = 75:25 (weight ratio)) as a positive electrode active material and 5% by weight of LiPAA (Lithium Polyacrylate) as a binder. The positive electrode slurry composition was applied to an aluminum current collector and dried to prepare a positive electrode. The loading of the manufactured positive electrode was 3.5 to 4.5 mAh/cm ² .

Lithium metal was used as the cathode.

After placing the anode and the cathode face to face and interposing a polyethylene separator with a thickness of 16 ㎛ and a porosity of 46 vol% between them, the electrolyte solutions prepared in Examples 2 and 3 and Comparative Example 1 were injected, respectively, to form a pouch cell. A lithium-sulfur battery was manufactured.

A pouch cell type lithium-sulfur battery has an electrolyte that is not exposed to air.

Specifically, the electrolyte solution prepared in Examples 2, 3 and Comparative Example 1 was used and stored for 1 week, 2 weeks, 3 weeks, and 4 weeks, and a lithium-sulfur battery in the form of a pouch cell injected with the electrolyte solution was used. The capacity maintenance rate was evaluated, and this was repeated for 4 weeks.

The capacity retention rate was calculated based on the 4th discharge capacity by running 4 cycles of 0.1C discharge/charge before storage, and the 4th discharge capacity was calculated by running 4 cycles of 0.1C discharge/charge after storage. The results are shown in Figure 2. indicated.

From the results of Figure 2, Comparative Example 1, which does not include lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, showed a significant decrease in capacity retention as the high temperature storage period of the electrolyte solution increased.

Example 2, where the molar concentrations of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide are the same, and the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is lithium bis. Example 3, where the molar concentration of (fluorosulfonyl)imide was higher, showed that the capacity was maintained even when the electrolyte was stored at high temperature for a long period of time.

In Experimental Example 2, when the electrolyte solution was sealed and stored in a high temperature chamber at 60°C for 4 weeks, most of the lithium bis(trifluoromethanesulfonyl)imide remained, but lithium bis(fluorosulfonyl)imide All resulted in decomposition.

Therefore, it can be expected that lithium bis(fluorosulfonyl)imide was decomposed over time in the results of Experimental Example 3, and accordingly, the concentration of lithium bis(trifluoromethanesulfonyl)imide was higher. It can be seen that Examples 2 and 3 showed excellent capacity retention rates.

From the above results, it was found that the electrolyte solution for lithium-sulfur batteries of the present invention has excellent storage stability at high temperature (60°C), and thus can improve the capacity retention rate of lithium-sulfur batteries containing it.

Experimental Example 4. High temperature (45°C) performance evaluation of coin cell type lithium-sulfur battery

Lithium-sulfur cells in the form of coin cells were injected with the lithium-sulfur battery electrolytes of Examples 1 to 4 and Comparative Example 1 in the same manner as in Experimental Example 3, except that lithium metal was applied to a copper current collector and used as a negative electrode. A battery was manufactured.

In coin cell-type lithium-sulfur batteries, the electrolyte is exposed to air and undergoes oxidation.

For the coin cell type lithium-sulfur batteries injected with the electrolytes for lithium-sulfur batteries of Examples 1 to 4 and Comparative Example 1, a charge/discharge measuring device (LAND CT-2001A, manufactured by Wuhan) was used. The lifespan characteristics were evaluated using

Specifically, discharge until 1.8V at a current density of 0.1C at 45℃, repeat charging 2.5 times until 2.5V at a constant current, and then repeat discharging and charging 3 times at a current density of 0.2C. Afterwards, the cycle was performed at a current density of 0.5C to measure the lifespan characteristics, and the results are shown in Figure 3.

From the results in FIG. 3, Comparative Example 1, which does not include lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, showed poor lifespan characteristics at 45°C.

Example 1 includes lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide as lithium salts, but the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is It contained a lower molar concentration than lithium bis(fluorosulfonyl)imide, showing poor lifespan characteristics similar to Comparative Example 1.

Example 2 showed that the molar concentrations of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide were the same, and the lifespan characteristics were improved at 45°C.

Example 3 contains a higher molar concentration of lithium bis(trifluoromethanesulfonyl)imide than that of lithium bis(fluorosulfonyl)imide, and Example 4 contains lithium bis(trifluoromethane). Containing only sulfonyl)imide, both Examples 3 and 4 showed improved lifespan characteristics under 45°C conditions and showed better results than Example 2.

From the above results, it can be seen that the lithium salt contains lithium bis(trifluoromethanesulfonyl)imide, or the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is the mole of lithium bis(fluorosulfonyl)imide. It was found that improved lifespan characteristics can be obtained at high temperature (45°C) when the concentration is higher.

Experimental Example 5. High temperature (45°C) performance evaluation of pouch cell type lithium-sulfur battery

A lithium-sulfur battery in the form of a pouch cell into which the electrolytes for lithium-sulfur batteries prepared in Examples 2 to 4 and Comparative Example 1 were injected was prepared in the same manner as in Experimental Example 3.

A pouch cell type lithium-sulfur battery has an electrolyte that is not exposed to air.

Lifespan characteristics were evaluated under the same conditions as in Experimental Example 4, and the results are shown in FIG. 4.

The results in FIG. 4 showed the same tendency as the results in FIG. 3 above. However, since the electrolyte of the pouch cell type lithium-sulfur battery was not exposed to air and did not oxidize, it showed better performance at high temperatures than the coin cell type lithium-sulfur battery.

Experimental Example 6. Evaluation of solubility of nitrogen compounds according to the type of organic solvent

The solubility of nitrogen compounds in ether-based solvents and carbonate-based solvents was evaluated using lithium nitrate (LiNO ₃ ) as follows.

As an electrolyte using an ether-based solvent, the electrolyte according to Example 4 prepared above was prepared. Next, as Comparative Example 2, the organic solvent was changed to ethyl carbonate (EC):dimethyl carbonate (DMC) (1:2 v/v) instead of 2-MeF:DME (2:8 v/v). Except that, an electrolyte solution was prepared according to the same method as in Example 4.

A photograph of the prepared electrolyte solution is shown in Figure 5 (left: Example 4, right: Comparative Example 2)

In addition, when preparing Example 4 and Comparative Example 2, lithium salt was dissolved in an organic solvent prepared at room temperature (25°C), and lithium nitrate was added. At the point when lithium nitrate precipitates without dissolving, the lithium nitrate The input amount was measured. The measured lithium nitrate content was expressed as solubility in 100 g of organic solvent, and the results are shown in Table 3 below.

organic solvent Solubility (g/100 g) Example 4 2-MeF:DME (2:8 (v/v)) 9 Comparative Example 2 EC:DMC(1:2 (v/v)) 0.2

Referring to the results of FIG. 5 and Table 3, it was confirmed that when the electrolyte solution for a lithium secondary battery contains a nitrogen compound, it is preferable that the organic solvent contains an ether-based solvent in order to utilize the nitrogen compound.

Claims (22)

Contains lithium salts, nitrogen compounds and organic solvents,
The molar concentration of the lithium salt is 0.5M to 0.75M,
The lithium salt includes lithium bis(trifluoromethanesulfonyl)imide,
The content of the lithium bis(trifluoromethanesulfonyl)imide is 50 mol% or more based on the total number of moles of the lithium salt,
The organic solvent is an electrolyte solution for a lithium secondary battery, characterized in that it contains 2-methylfuran and linear ether in a volume ratio of 5:95 to 20:80. delete delete In claim 1,
The lithium salt further includes lithium bis(fluorosulfonyl)imide,
An electrolyte for a lithium secondary battery, wherein the molar concentration of lithium bis(trifluoromethanesulfonyl)imide is equal to or higher than the molar concentration of lithium bis(fluorosulfonyl)imide. delete delete In claim 1,
The linear ether includes dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethylmethyl ether, ethylpropyl ether, ethyltertbutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, Diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol di. Vinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tert-butyl ethyl ether and ethylene glycol ethyl methyl ether. An electrolyte solution for a lithium secondary battery, characterized in that it contains at least one member selected from the group consisting of. In claim 1,
The organic solvent is 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3- Dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-diok Solan, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3- 1 selected from the group consisting of dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,4-dimethoxy benzene and isosorbide dimethyl ether An electrolyte for a lithium secondary battery, characterized in that it further contains more than one species. In claim 1,
The organic solvent is an electrolyte solution for a lithium secondary battery, characterized in that it exhibits a solubility of 2 g/100 g or more for the nitrogen compound at room temperature based on 100 g of the organic solvent. In claim 9,
An electrolyte solution for a lithium secondary battery, wherein the room temperature is in the temperature range of 20°C to 35°C. In claim 1,
An electrolyte solution for a lithium secondary battery, wherein the organic solvent does not contain a carbonate-based solvent. In claim 11,
The carbonate-based solvent includes dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, An electrolyte solution for a lithium secondary battery, characterized in that it is 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, a halide thereof, or a mixture of two or more thereof. In claim 1,
An electrolyte solution for a lithium secondary battery, wherein the nitrogen compound includes a nitric acid compound or a nitrite-based compound. In claim 1,
The nitrogen compound is an electrolyte for a lithium secondary battery, characterized in that it is contained in an amount of 2% to 10% by weight based on the total weight of the electrolyte for a lithium secondary battery. In claim 1,
The electrolyte for a lithium secondary battery is characterized in that more than 90% by weight of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide remains when stored at a temperature of 45°C or higher. In claim 15,
The electrolyte for a lithium secondary battery is a lithium secondary battery, characterized in that 90% to 98% by weight of the initial weight of the lithium bis(trifluoromethanesulfonyl)imide remains when stored for 4 weeks at a temperature of 45°C or higher. Electrolyte for batteries. In claim 15,
An electrolyte for a lithium secondary battery, characterized in that the storage temperature is 45°C to 65°C. anode; cathode; A separator interposed between the anode and the cathode; And a lithium secondary battery containing an electrolyte,
A lithium secondary battery, wherein the electrolyte is the electrolyte of any one of claims 1, 4, and 7 to 17. In claim 18,
The positive electrode is a lithium secondary battery characterized in that it contains a sulfur-containing compound as a positive electrode active material. According to clause 19,
The sulfur-containing compound is inorganic sulfur (S ₈ ), lithium polysulfide (Li ₂ Sn, 1≤n≤8), carbon sulfur polymer (C ₂ Sx)m, 2.5≤x≤50, 2≤m) or any of these. A lithium secondary battery comprising a mixture of two or more. In claim 18,
A lithium secondary battery, wherein the negative electrode contains lithium metal, lithium alloy, or a mixture thereof as a negative electrode active material. In claim 18,
A lithium secondary battery, characterized in that the lithium secondary battery is a coin-type battery or a pouch-type battery.

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